Liquid tanks, when they are likely to be moved with the liquid that they contain, are generally equipped with a ventilation system that guarantees environmental safety when the tank is subjected to various stresses: movements in any direction and of any amplitude, thermal stresses, underpressure and overpressure.
This requirement is encountered in the case of fuel tanks, in particular when they are mounted on motor vehicles and when it is essential to prevent liquid fuel from getting out and to manage large gas pressure and volume changes when filling the tank and throughout the time that the liquid is stored in said tank.
Solutions have been developed to solve these problems that use safety valves immersed in the tank, the upper part of the valves passing through a wall of this tank. These valves generally open into a duct (or a set of ducts) leading to a box or canister containing a substance capable of trapping the liquid vapors present in the gases originating from the tank. This assembly of valves and ducts (and also optionally the canister) generally constitutes what is known as the ventilation system of the tank.
However, it is not uncommon that systems of this type have further difficulties due to the fact that, because of the particular operating circumstances, such as sudden movements or excessively high angle of tilt of the vehicle, liquid originating from the tank may nevertheless pass through the barrier of the safety valve and get into the duct leading to the canister, or even reach the latter and impair its performance.
To solve this accidental carryover of liquid out of the tank, it has been sought to retain the liquid escaping via the duct leading to the canister by interposing, in this duct, a dead volume intended to act as a vessel for collecting the liquid and for letting the vapors pass freely through. This dead volume, which is therefore part of the ventilation system, is generally known as an LVS (or liquid/vapor separator).
There are mainly two types of LVS: those referred to as “passive”, which comprise in their base a simple valve that opens under the effect of the weight of liquid that they contain; and those referred to as “active”, which are drained by means of an active device (a pump, for example a jet pump). The first (passive) type is of course only suitable for LVSs located in the top of the FTs, preferably above the maximum fill level or in any case: that are normally above the level of liquid in the tank. The second (active) type is also suitable for the LVSs known as “submerged” LVSs, i.e. those in which the level of liquid in the tank may reach or even exceed the level of liquid contained in the LVS.
An LVS of passive type is for example known from Patent EP 1 020 670 in the name of the Applicant. In the latter, the liquid is only discharged when the level of fuel in the tank has dropped below the low point of the LVS and when a certain amount of liquid is present in its internal volume (in order to be able to open the valve, which is at said low point, by gravity).
An LVS of active type is for example known from U.S. Pat. No. 6,698,475, where the pump that drains the LVS is a jet pump which may either be activated by a fuel return, or by a by-pass of the main pump for supplying the engine with fuel. In order to prevent, when the LVS is submerged and when the pump is at rest, liquid from going back into the LVS under the effect of hydrostatic pressure, the line connecting the LVS to the jet pump is equipped with a check valve.
Such a valve generally only opens under the effect of a certain pressure threshold (hydrostatic pressure of the liquid on top of it and/or vacuum created by the drain pump) and its sealing is even better when this threshold is high. The jet pump must therefore be sufficiently powerful to overcome this pressure threshold. In addition, a certain time is generally required in order to reach this threshold, which may impair the efficiency of the system when the vehicle is started. Finally, when the pump is at rest, the valve is closed and therefore liquid could be trapped in the LVS when the vehicle is at rest (for example, following condensation caused by a drop in temperature), possibly even in a relatively large amount, which would not be rapidly discharged when the vehicle is restarted.